texts: Has any moon achieved "retrograde equatorial orbit"?

texts:
There are many moons which have low (almost negligible) inclination and can be considered to rotate at the planet's equatorial plane. For instance, Galilean moons have almost negligible inclination (< 1$^\circ$) and so they are considered to have a "prograde equatorial orbit".
OTOH, "Retrograde equatorial orbit" is that orbit which has an inclination of exactly 180° (or near 180°). Has any moon achieved such kind of orbit? I have found some Saturnian moons (or moonlets) which have inclination around 177° and didn't found anything beyond that. Why is it hard to achieve a "Retrograde equatorial orbit"?

Tangentially related questions: Can a moon rotate perpendicular to planet's plane? [90° inclination] (Theoretically yes, practically hard to achieve; See Kozai Mechanism)

Physics.SE
WorldBuilding.SE

Also related:  Are all satellites of all planets in the same plane?


texts: Can I calculate the apparent position of comet NEOWISE C/2020 F3 from a topographic position with Skyfield?

texts:
I'd like to get .altaz(next_several_weeks) of this comet from Skyfield and .separation_from() other planets.
It reads the JPL development ephemerides and can import star catalogs, but is there a way to add some orbital elements for a given comet or asteroid from some standard database and at least approximately propagate them to get an astrometric position and and all the other goodies Skyfield can provide from there?
If so, are these the best places to find the appropriate orbital elements for comet NEOWISE C/2020 F3?

https://minorplanetcenter.net/mpec/K20/K20G05.html

this answer

find_orb (from this answer)

From Horizons:

JPL/HORIZONS                 NEOWISE (C/2020 F3)           2020-Jul-09 02:25:23
Rec #:90004465 (+COV) Soln.date: 2020-Jun-16_22:44:15      # obs: 353 (70 days)

IAU76/J2000 helio. ecliptic osc. elements (au, days, deg., period=Julian yrs):

  EPOCH=  2458958.5 ! 2020-Apr-19.0000000 (TDB)    RMSW= n.a.
   EC= .9991762343449366   QR= .2946766901808353   TP= 2459034.1806295365
   OM= 61.00968684540688   W= 37.27670558969492    IN= 128.9375082086831
   A= 357.7190774701175    MA= 359.9889750746684   ADIST= 715.1434782500542
   PER= 6765.8344565066    N= .000145677           ANGMOM= .01320322
   DAN= .32818             DDN= 2.87461            L= 35.4449114
   B= 28.1060796           MOID= .362501           TP= 2020-Jul-03.6806295365

Comet physical (GM= km^3/s^2; RAD= km):
   GM= n.a.                RAD= n.a.
   M1=  10.9     M2=  14.9     k1=  17.25  k2=  5.      PHCOF=  .030

COMET comments
1: soln ref.= JPL#10, data arc: 2020-03-27 to 2020-06-05
2: k1=17.25, k2=5., phase coef.=0.03;


texts: Exoplanet radius density relation according to NASA exoplanet archive

texts:
I downloaded the info from NASA exoplanet archive at: https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=compositepars

I named the file "NASA.Composite.Planet.Data.csv". I had to edit the file to remove the header which was preventing the program below from running. (I put the header at the top of the program below)

I then plotted it with Matplotlib using this python (anaconda) program: https://planet.wikia.com/wiki/User:Granpa/Python_program

I got this image:
https://i.imgur.com/1057YvQ.png

Can anyone explain what I am seeing


texts: How impermeable are the surfaces of the moon?

texts: In the event that we want to dig into the surface of either face of the moon do we know yet if the surfaces of the moon are particularly difficult or particularly easy to dig in?


texts: Future redshifting and effect on the 'pitch' of CMB radiation

texts:
After discovering this question exploring the sound of a blackbody, I started wondering about the sound of the Cosmic Microwave Background radiation from the Big Bang, specifically what the current pitch might be at its peak frequency and how that might evolve over time due to redshifting. Here's what I've been able to deduce so far:

$$
pitch_{CMB} = 160.4\,GHz \approx D_{33} = 157.7\,GHz
\\
pitch_{CMB}\,lowered\,by\,30\,octaves = {160.4\, GHz \over 2^{30}} = 149.38\, Hz \approx D_3 = 146.83\,Hz
$$

Building on this result:

How much time will need to elapse before the CMB redshift will cause this pitch to drop by an octave (or even by a semitone)?
How much variability would this pitch have depending on the dipole anisotropy of the CMB from our perspective as well as due to more generalized anisotropies in the CMB?
What pitch would the peak value of this blackbody have had at earlier points in the history of the Universe?

I've tried to apply some of the information in this question and elsewhere, but would greatly appreciate hearing from someone with more expertise in the subject area if this unorthodox comparison has piqued your interest like it has mine.


texts: could dark energy be a force coming from the hyperspace outside the universe?

texts: It seems to me that the expansion of the universe is only being looked at one way. a repelling force, basically being the opposite of gravity.  After pondering this for many nights, I began to wonder if its possible that the changing pressure outside the universe could cause the expansion thst we observe.


texts: What is the final destiny of a neutron star?

texts:
As I understand, neutron stars are born as extremely bright, extremely fast spinning cores of stars dying in a supernova. However, several websites tell me that within a course of a few years, the surface temperature of a neutron star falls from several trillion kelvin to only a few million kelvins. Furthermore, with the passage of time, the spinning speed of the neutron star also decreases considerably.

This raises the question: what is the final destiny of a neutron star? Does it always stay so horribly magnetic, hot and fast spinning or does it keep degrading into some form of cold, extremely dense star core with a much weaker magnetic field or do some of its features (specially the magnetic field strength and spin) stay at hightened levels forever (or at least several hundreds of billions of years)?


texts: Last possible total solar elipse in the Earth's history

texts: So the Moon is drifting away from Earth by 3.8 cm a year due to tidal interactions. With the Moon slowly receding from Earth, there will come a time when it no longer has sufficient angular size to cover the Sun completely. When will the last total solar eclipse occur?(when will the Moon's umbra no longer be able to project onto the Earth?)


texts: Is there an established distinction between a geyser and a cryovolcano in solar system bodies?

texts:
Is there an established distinction between a geyser and a cryovolcano in the context of cold solar-system bodies such as the moons of Jupiter and Saturn, or Kuiper belt objects (e.g. Pluto)?

Both can lead to spreading of new water across the surface, and both can result in plumes of water vapor above the surface. Do they have distinctly different mechanisms?

Or is it unclear or in a state of flux, a bit like the diagram in @zephyr's answer to Do astronomers generally agree that the distinction between comets and astroids is not so clear?


texts: Why do X-ray binaries such as GRS-1915 have active and quiet states?

texts: X-ray binaries such as GRS-1915 feature a black hole and a regular star. The x-ray luminosity is likely related to the rate of mass flow into the black hole from the star so when there is a lot of mass flow the x-ray luminosity is high but this explanation seems to just beg the question as to why the mass flow would change over time.


texts: Why is declination positive in the northern hemisphere?

texts:
Simple question -- why is declination positive in the northern hemisphere?

Seems like it should've been called "inclination" since the word originated north of the equator.


texts:
Could Hawking radiation be detected with instruments available today? If not, what sort of instruments would be required to observe it? Would there be any way to confirm through observations the existence of Hawking radiation?

I'm primarily asking about techniques that we could reasonably implement with current technology or something that could be implemented in the near future, but if you need to get close to a black hole to be able to observe the radiation and couldn't do it from a spacecraft in Earth or solar orbit, then that would be the answer.


texts: How are galaxies moving so much and why are they moving?

texts:
I don't understand and I haven't understood for a while.
How is it that all the galaxies in the universe are moving, how they are moving and how fast?


texts: What happened before, Big Bang or inflation?

texts:
I always thought that the cosmic inflation happened after the big bang (10-36 sec) and lasted for a very tiny fraction of time).
Now, I recently came across a couple of articles (links below) claiming that Big Bang happened after the inflation stopped and when the reheating phase started.
I'm a bit confused as I always imagined the Big Bang as "when everything started", so how can something be happened before that?

Ethan Siegel (2019) "What Came First: Inflation Or The Big Bang"
Gabriele Veneziano (2017) "A Quantum Universe Before the Big Bang(s)?"


texts: Can air pollution affect the observed positions of stars?

texts:
I live in a big suburban area where pollution is quite high, and I have been testing a sextant by measuring the angle between pairs of stars. I compared the measured angle with the one obtained from ephemerides, where the latter takes account of the alteration of the body altitude due to refraction in the atmosphere.
For instance, I measured the angle between Castor and Pollux, both at apparent altitudes of ~ 39 degrees.
The comparison works pretty well, but the sextant appears to systematically overestimate the angle by ~ 1'. Can the air pollution in my area (not light pollution) alter the refraction index of the surrounding air mass and thus be responsible for this discrepancy?
Thanks!


texts: Can the theory of multiverse explain dark matter?

texts: If there are many universes, they must have a gravity effect on each other. Since dark matter is measured through gravity, but we don't know the mass behind it, isn't it possible that it comes from another universe?


texts: What is that donut-shaped object I see in my telescope?

texts: I was looking through my telescope for the first time and came across this weird star. I think its a celestial donut but im not sure.


texts: Neutron star size in different reference frames

texts:
What is the estimated size of neutron stars observed in their reference frame and in our reference frame?

That is, how bent is space-time around neutron stars?


texts: Can two neighboring galaxies move apart at steady speed?

texts: While I was trying to understand the three models that obey Friedmann's two assumptions of a non-static universe, I came across a line that says and I quote "It (referring to Big Bang) starts at time zero and eventually the galaxies are moving apart at steady speed." The question is - is it possible for two massive space bodies to move apart at a 'constant' speed? What do you say on account of the gravitational forces acting between the galaxies? And does it mean the universe will only expand and never come to rest? P.S. I am assuming 'steady' to mean 'constant'.


texts: Does time dilate during night time when earth orbits around the sun?

texts:
Number of the solar middays (noon) = Number of the solar midnights (ALWAYS). The diagram (Top view but not to the scale) in the following link depicts the path traced by the earth in its orbit (either circular or elliptical) around the sun.

https://i.postimg.cc/C5K2wX6V/IMG-3066.jpg
Both Midnight and Midday happen simultaneously and instantly. Any point on the outer circle represents solar midnight while on the inner circle solar noon. It's quite clear that the length (circumference) of the outer circle is greater than the length (circumference) of the inner circle. This means after the completion of one year, the occurrences of the Midnights are more the occurrences of Middays.
Earth not only rotates around its own axis but also revolves around the sun in its orbit however both axial and orbital motion of earth independent of each other. There is a possibility that the earth may glide w/o rotation for a short while and time dilate but such things never noticed either.


texts: How strong is the gravity from half of the distant universe?

texts: I would like to know, how strong would the gravity be from the distant universe if we were somehow able to remove half of it.  For example, if everything behind us did not exist, how strongly would the stuff in front of us be pulling?  By distant, let us consider everything beyond 10 billion light years (e.g. remove all the closer stuff).  How many G's acceleration would I experience (discounting the fact that I would be in free fall and wouldn't feel it)?


texts: What type of telescope can show cliffs on the moon surface from a city location?

texts:
I am a complete novice in this world and I would like a telescope where I could have enough power to view the moon's surface, I don't expect to see the flag or the hover sending signs to me, but perhaps enough to view cliffs, and other details from the surface of it and perhaps from other places.

I honestly don't even know what kind of telescope to purchase. I live in a capital city, so there is a lot of light around here. My apartment is high with a balcony, which is where I would place it and there is nothing in front of me from there.

What kind of telescope would fit my needs?
Are there reputable online stores that I could consult?

I have a $500 budget for this and honestly, I have no clue about specs or anything, I am a completely novice in this field.


texts: Can't find Cassiopias Schedar or Caph in Tycho-2 catalog supplement 1

texts:
I'm trying to locate three stars inside Cassiopeia, gam cas, Schedar, and Caph. I have found gamma Cas inside the Tycho-2 catalog supp1.dat file, however when I look for Schedar or Caph I have no luck. I converted their RA and dec to decimal degree, Schedar = (10.12, 56.53) and Caph = (2.29, 59) and used the Vim command

:/|002\.

to find their location in the star catalog, but I haven't found any stars that match the RA, dec, or even visual brightness, which are 2.24 for Schedar and 2.28 for Caph. I'm kind of stuck at this point and not sure where to head from here.


texts: Is it really possible for material such as dust, gas and other stellar debris that come near the black hole, doesn't fall into it, instead forms a flattened band of spinning material?


texts: Could life forms have travelled between the Earth and Mars on meteoroids?

texts:
Mars meteorites have been found on Earth, and there likely are Earth meteorites that have crashed on Mars.
If you take any random piece of rock from the surface of Earth, the odds are there will be some microbes on it. Some microbes from Earth can survive as they travel through space (see the Surveyor III part returned by Apollo).
Could unicellular life forms end up on Mars after being transported by an Earth meteoroid? Would it be possible for some to survive the trip to Mars? What about through the martian atmosphere?


texts: Could different views of the universe simply be different points in time?

texts:
I'm not sure if this is the correct stackexchange site for this question.

I've been reading this article on Vice Motherboard There’s Growing Evidence That the Universe Is Connected by Giant Structures which while admittedly simplified for simpler folk like me put me wondering:

Is it at all possible that when we look out in to the distant universe in one direction, we are seeing the same galaxies and stars at a different point in time than if we look out in another direction? Could this occur if the distant body is moving laterally at a fast enough velocity relative to our position, or if the shape of the universe itself loops around?

Or is this not a proper interpretation of current theories of cosmology?


texts: Could dark matter compress and form black holes? Since dark matter is even more abundant than normal matter, a dark matter black hole should not be rare...right?


texts: How common or rare is it to find stars outside of a galaxy? Can such a star still have planets that contain life? Recently, I think that I heard of a star with a planet that entered our Galaxy. Did this star come here from another galaxy or was it originally from the Milky Way?


texts: Can "rogue" supermassive black holes be made this way?

texts: Could two galaxies (one big and one small)intersect at a velocity to allow the smaller black hole to escape but not the galaxy around it?


texts: Covariance matrix not calculated when fitting gaussians?

texts:
So I am using Astropy's basic models.Gaussian1D model and a fitter = fitting.LevMarLSQfitter() to fit 3 Gaussian pulses with some added random noise. The fit is good, but when I type

fitter.fit_info, according to  documentation here, it should automatically calculate covariance matrix and add it to the fit_info, but it doesn't, fit_info['param_cov'] is simply None. What is going on?


texts: What is the minimum size of a ball of gas to become a star?

texts:
I know there are two criteria to meet in order for nuclear fusion to occurs.

High temperature (many times temperature at Sun's core)
High pressure (protons are very close to each other)

[Goal]

However I want to know what equations allows me to calculate the amount of hydrogen gas needed so that the internal pressure can produce the right amount of heat for nuclear fusion to occur.

[My own understandings]

I also understand that the temperature of our Sun's core is not sufficient to overcome the coulomb barrier, it make use of quantum tunneling to achieve fusion instead.

[Question]

What is the minimum amount of hydrogen gas required to form a star? (also please include the name of the equation used)

[Assumptions]

shape is perfectly spherical
no angular velocity, not rotating
homogenous, consist of 95% hydrogen and 5% helium
sustain fusion for at least 1 million years


texts: Tunnel vision while viewing through a reflector

texts:
I have a Celestron astromaster130 eq. While viewing through its eyepiece, or using a smartphone camera for photography purpose there is always a definitive no light region( it is circular in curvature). I make sure that I look directly at the eyepiece, but this tunneling makes it really difficult to view things at times. I have googled this, but am not sure of what this effect is called. Is it a general problem or is there a need to rectify some fault in the telescope itself. If not, what are the ways in which this can be effectively reduced?
Thanks.
Edits are most welcome.


texts: I have read before that Jupiter is named after the great Roman god because of how bright it is. I can't help but wonder, why wasn't the name given to Venus instead? Isn't Venus much brighter than Jupiter in the night-sky, or am I mistaken?


texts:
In general, almost everyone describes Earth as an perfect sphere, but in reality it isn't a perfect sphere.

Is there any specific reason why Earth is not perfect sphere ?


texts: Is the smaller companion of Pulsar J0453+1559 still the smallest known neutron star? Was it further confirmed to be a NS after Martinez et al. (2015)?

texts:
@ProfRob's Physics SE answer to What is the theoretical lower mass limit for a gravitationally stable neutron star? was linked just now in their answer to Can a neutron star ever be less than about 1.44 solar masses (the Chandrasekhar limit)? Why not? which notes that a smaller neutron star companion to the J0453+1559 pulsar might (or might not) be the smallest currently known pulsar:

A number of neutron stars in binary systems have measured masses below $1.44 M_\odot$ (e.g. a pulsar of mass $1.251 \pm 0.021 M_\odot$, McKee et al. 2020). I think the current lowest mass contender is $1.174 \pm 0.004 M_\odot$ (Martinez et al. (2015) Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry.

The abstract of Martinez et al. (2015) includes the sentence:

If this companion is also a neutron star (NS), as indicated by the orbital eccentricity of the system (e=0.11), then its mass is the smallest precisely measured for any such object.

and Section 3, Results explains:

Given the orbital eccentricity (e = 0.11), the companion is very likely to be a neutron star – if it had evolved into a massive white dwarf star there would be no sudden mass loss associated with a supernova explosion and the system would have retained the circular orbit that is characteristic of compact accreting systems. This is consistent with the non-detection of an optical counterpart of the system in any of the optical catalogs. However, the high eccentricity does not entirely settle the matter: the recent discovery of a recycled pulsar with a massive (∼ 1M⊙) companion PSR J1727−2946 Lorimer et al. (2015) and an orbital eccentricity of 0.0456 bridges the previously observed eccentricity gap between systems with NS and massive WD companions. In the remainder of this paper we will assume, with caution, that the companion is a NS.

That the companion may be a neutron star is important for this analysis; the next sentence is

The pulsar’s orbital eccentricity allows a detection of the advance of periastron, ˙ω. If
both components are compact, as implied by the optical non-detection, this is given by:
$$\dot{\omega} = \dot{\omega}_{GR} + \dot{\omega}_{K}$$
[...]

And the conclusion section notes that this is a relatively small eccentricity for double neutron stars listed in Table 1 as of 2015.

It's now been six years, so I'd like to ask:
Question: Has the smaller companion to Pulsar J0453+155 been confirmed to be a neutron star by any further work beyond that of Martinez et al. (2015) and is it still the smallest known neutron star?
Has the smaller companion to Pulsar J0453+155 been confirmed to be a neutron star after Martinez et al. (2015) and is it still the smallest known neutron star?


texts: Poisson's equation-like formula in axis-symmetric Galactic potential in terms of Oort constants

texts:
I am tackling this problem, from a 2016 Cambridge Astrophysics Tripos past paper:
Let $\Phi(R, z)$ be the axi-symmetric Galactic potential. At the Solar location, $(R, z) = (R_0, 0)$, prove that
$$\frac{\partial^{2} \Phi}{\partial z^{2}}=4 \pi G \rho_{0}+2\left(A^{2}-B^{2}\right)$$
where $G$ is the gravitational constant, $\rho_0$ is the density in the Solar neighborhood and $A$ and $B$ are Oort’s constants.
What I do is:
Rewrite equation we want to prove
Note:
$A^2-B^2=-\frac{v_c}{R}\frac{\mathrm{d}v_c}{\mathrm{d}R}$
Using definiton of $A$ and $B$. (Which are: $\left.A \equiv \frac{1}{2}\left(\frac{v_{c}}{R}-\frac{d v_{c}}{d R}\right)\right|_{R_{0}}$ and $B \equiv-\left.\frac{1}{2}\left(\frac{v_{c}}{R}+\frac{d v_{c}}{d R}\right)\right|_{R_{0}}$).
Rewrite what we want to prove:
$$
\bbox[5px,border:3px solid green]{
\frac{\partial^{2} \Phi}{\partial z^{2}}=4 \pi G \rho_{0}-2\frac{v_c}{R}\frac{\mathrm{d}v_c}{\mathrm{d}R}
}
$$
Use Poisson's equation:
$$\nabla^2\Phi=4\pi G\rho_0$$
Expand LHS of Poisson:
$$\frac{\partial^2}{\partial R^2}\Phi+\frac{\partial^2}{\partial z^2}\Phi=4\pi G \rho_0$$
Use equation of motion for circular orbit:
$$\frac{v_c^2}{R}=\frac{\partial}{\partial R}\Phi$$
Differentiate once more wrt $R$:
$$\frac{\partial^2}{\partial^2 R}\Phi=\frac{\partial}{\partial R}\frac{v_c^2}{R}=\frac{2v_c}{R}\frac{\mathrm{d}v_c}{\mathrm{d}R}-\frac{1}{R^2}v_c^2$$
Collect results
Substitute result into Poisson's equation expanded form:
$$\frac{2v_c}{R}\frac{\mathrm{d}v_c}{\mathrm{d}R}-\frac{1}{R^2}v_c^2+\frac{\partial^2}{\partial z^2}\Phi=4\pi G\rho_0$$
Rearrange:
$$
\bbox[5px,border:3px solid green]{
\frac{\partial^2}{\partial z^2}\Phi=4\pi G\rho_0-2\frac{v_c}{R}\frac{\mathrm{d}v_c}{\mathrm{d}R}+\bbox[5px,border:3px solid red]{\frac{1}{R^2}v_c^2}
}$$
Conclude
Compare with what we wanted to prove (ie the two green-boxed equations). We have an extra term (red boxed) which shouldn't be there. How do I fix the proof?


texts:
I'm having trouble understanding the basic model for how matter bends space. The classic illustration is the canvas and the ball but that model needs a reference frame where one can define a downward direction to work. For the life of me I can't figure out why matter just doesn't "sit" in space.
I'm sorry this question isn't better formulated, maybe someone can point me towards a video or something?


texts: How can we be 13.2 billion light years from another galaxy?

texts:
How did we get 13.2 billion light years from the faraway galaxies being discovered with Hubble?

If we all started near each other at the Big Bang, and we all travelled slower than the speed of light, and the universe is about 13.9 billion years old, how could we possibly be 13.2 billion light years away from recently discovered galaxies?

I know "inflation" was supposedly superluminal, but I bet no one thinks that matter dispersed several billion light years apart during that operation.


texts:
When a planet is spinning around its own axis, it has an effect on the trajectory of its satellites. I believe it is called frame dragging.

Spin increases the kinetic energy of an object, contributing to its mass-energy, but some other effect must come into play as well.

How does one differentiate between spin and orbit velocity?

What is the effect of the spin? Lets say that Earth suddenly started spinning faster, what would happen to the moons trajectory?


texts: Is the "mean orbit" centered on ellipse center or ellipse focus?

texts:
Mean longitude and mean anomaly of a body are calculated as the angle covered by the body along a fictious circular orbit, and this is ok.
But where is this orbit centered? Five different sources say five different things:
Wikipedia: circle center = ellipse focus

University of Texas: who knows?!? No ellipse at all:

University of California: who knows? Mean anomaly mentioned but not drawn; only eclipse shown, no circle:

Britannica Enciclopedia: who knows? Both circle and ellipse shown, but mean anomaly not shown:

Max-Planck-Institut für Sonnensystemforschung: circle centered on ellipse center

Theory & formulas: longitudes and anomalies summed up together, no pictures at all.
I am very confused.


texts: What percentage of the celestial sphere can the Moon cover?

texts:
I was solving a task that said (paraphrasing):

  What percentage of the night sky can the Moon cover during the entire
  year, when observed from all points on Earth (what percentage of the
  night sky can be studied with the occultation method)?

This is a task I read from a book, and the solution states that the Moon covers a "ribbon" of width $\alpha$ where $\alpha = 0.5 ^{\circ} + 2*1^{\circ} + 2*5.3 ^ {\circ} = $ angular diameter of the Moon + 2*the Moon's daily parallax + 2*the inclination of the Moon's orbit to the ecliptic; and length $360 ^{\circ}$.
Why is this like this, I understand the diameter and parallax part, but I couldn't visualize the problem very well. Why is the third number the inclination to the ecliptic and not to the equator?


texts: SIMBAD TAP queries seem to indicate there are no distance measurements for a great many nearby stars, including Alpha Centauri or Barnard's Star

texts:
I'm attempting to create a map of all objects within 50LY, including main-sequence stars, binaries, planemos, etc. As such, I'm trying to do it all from first principles, building a micro-ETL to scrape SIMBAD data and build a model. However - I'm experiencing some issues with getting distance measurements from ADGL queries. Watch the following:
Using TAP (http://simbad.u-strasbg.fr/simbad/sim-tap)
SELECT * FROM BASIC WHERE main_id like '%alf%Cen%'
Returns 4 rows:

However - when I check out the mesDistance table with this query:
SELECT * FROM mesDistance WHERE oidref IN (3396297, 3396059, 3396054, 7880120)
Returns empty.... So maybe there's a missing parent. Let's check h_link
SELECT * FROM h_link WHERE parent IN (3396297, 3396059, 3396054, 7880120) OR child in (3396297, 3396059, 3396054, 7880120)
Which yields no new objects. 3396059 is the top level object and it has no distance measurement.
However, many other objects show up just fine, such as Proxima Centauri (OID = 3379714 )
It's VERY hit or miss where distance measurements exist and where they don't, but it makes it impossible to query objects based on relative distance. I'm guessing that maybe this has something to do with the age of the catalogs or studies which create a new distance measurement. If a catalog is old enough, it probably wasn't digitized or forced into the SIMBAD ADQL model...
Does anybody have some good suggestions on how to get distance data for nearby stars? Preferably something modern, or with the ability to join it to SIMBAD references. I'd like not to have to do a fuzzy join based on RA/DEC or scraping through alternate IDs. That can get messy or break table grain.


texts: Would a Einstein–Rosen bridge change size and/or position in an expanding Universe?

texts:
I appreciate that a Einstein–Rosen bridge has never been observed but since there are some valid theories in General Relativity I thought I'd ask this.

Would the start and end point of a Einstein–Rosen bridge change in an expanding Universe or would the whole thing stretch but retain it's start and end position?


texts: Create a source target and noise and stack images

texts:
I am a new one to learn who to stacking images and coding.
If I create a blank 1028*1028 pixels. I would like to ask how I can create a few artificial sources with peak value from 5000 to 10000 which is symmetric 2D Gaussian with a width sigma=3 pixel. And create a Gaussian random noise with sigma=2000 for each pixel (repeat 3 times ). Final to make staking all the 3 noise images and all signal+noise stacked images=?


texts:
Venus is somewhat lighter then Earth, yet has a much thicker atmosphere. One would imagine that the following should be true:

During the formation phase, all inner planets had captured as much gas as they could hold per gravitational/thermodynamic equilibrium. After all, even puny Mars managed to capture a sizable atmosphere.
The rate of atmospheric escape should be much higher for Venus:

Venus receives more heat from the sun, thus higher Jeans escape rate
Venus has negligible magnetic field, so some of its atmosphere must be lost to direct "blow-off" by solar wind

Yet, it is Earth that seemingly misses a great deal of atmospheric volume. So the question is: what are the current theories regarding the "thinning" of the Earthian atmosphere? When and why the atmospheric gases had left the planet?


texts:
Assume that a star has a uniform mass density. Then how does the temperature
scale as a function of the distance to the center?
Note: I am not looking for an answer that involves derivations or integrations.
Here is what I did:
The star is in equilibrium so the pressure at distance $r$ must equal the gravity force at distance $r$. Pressure is given by $P=nKT$, and so $P\times Area=F_g$.
Then $nKT \times 4\pi r^2=F_G$. The mass of the part of the star contained within radius $r$ from the center is $\frac{4}{3}\pi r^3\times m_sn$ where $m_s$ is the mass of each particle and $n$ is the number of particles per volume.
Then we can rewrite $nKT \times 4\pi r^2=m(\frac{G\frac{4}{3}\pi r^3\times m_snF}{r^2})$ and simplify to $KTr=\frac{m}{3}Gm_s$. Isolating $T$ we get that $T=\frac{mGm_s}{3Kr}$.
This seems to imply that the temperature decreases the further we are from the core, however I am not sure what $m$ should stand for in $F_g=mg$, should $m=m_s$, i.e be the mass of a single particle? Or does $m$ itself depend on the radius in which case the answer I got is invalid.
Can someone verify that the answer I got is right?


texts:
In this picture of 3C273, we can see a jet—estimated to be around 200,000 light-years long—being emitted.

I'm trying to find an estimate for the mass of the jet. It's a stream of electrons, accelerated through synchrotron radiation (see here). There seems to be lots of information on its spectroscopic properties and the mass of the quasar (886 million solar masses), but I can't find an estimate for the mass of the jet itself. Does anyone know of a source I could look at to give me some idea?


texts: If we lived in a multiverse, what would our universe most likely then be named?

texts:
We live in the Milky Way Galaxy, but we don't just call it "The Galaxy", because we know there are multiple different galaxies. So if we lived in a multiverse, it wouldn't make much sense to keep calling our universe "The Universe".
How are names for things like this created, and what would a likely name for our universe be?


texts:
The picture was taken with a smartphone camera at Latitude 48.860045 | Longitude: 2.366202 the 31th of July at 01h07min15s Paris time. On the left is obviously the Moon, and the lower red circle points the point of Latitude: 48.852886 | Longitude: 2.369167. (It's the angel of liberty, place de la Bastille, Paris.)

What does the upper circle point to ?

At the beginning I thought it was Venus, but as far as I remember, it disappears quite early in the evening. I don't think it is Mars : the light reflection on it isn't red enough.


texts: What was the the value of the Hubble constant at the time of the CMB's 'release' (i.e., 379,000 years after Big Bang)?

texts:
What about its value roughly 9 billion years after the Big Bang, when dark energy started to 'take over' and accelerate the expansion of the universe?
Is there a timeline or chart somewhere that shows approximate, theoretical values of Hubble's not-so-constant constant throughout the lifetime of the Universe?


texts: Can you simulate artificial gravity by using an electromagnetic field?

texts:
If you create a electromagnetic field under a surface could you then create artificial gravity effect by wearing magnetized clothing and shoes?

Or could you bombard objects that you want to experience gravitational forces with artificial gravitons or graviolis? Or inject or drink a ferrofluid (non-toxic) like that used in rocket fuel?

  A ferrofluid (portmanteau of ferromagnetic and fluid) is a liquid that
  becomes strongly magnetized in the presence of a magnetic field. A
  grinding process for ferrofluid was invented in 1963 by NASA's Steve
  Papell as a liquid rocket fuel that could be drawn toward a pump inlet
  in a weightless environment by applying a magnetic field.

  https://en.wikipedia.org/wiki/Ferrofluid

What I mean is create the forces of attraction and then make the objects you want to attract attractive, obviously.

This is as opposed to using other methods such as acceleration or centrifugal or centripetal force (or in addition to it).


texts: Recent research regarding if the local group of galaxies will remain bound to the Virgo Cluster?

texts:
The question of if the local group of galaxies will remain bound to the Virgo Cluster has been asked before (Is the local group bound to the Virgo cluster?). I am wondering if there is any more recent research regarding this? I haven't found anything more recent than the 2002 paper mentioned in the post I linked above: https://arxiv.org/abs/astro-ph/0204249
This is a subject that I know very little about, so I'm looking for advice on how to get more informed about it. There is a comment on the aformentioned post that says, "The confusing part is that there seems to be a gravitationally bound Virgo cluster, of which we are not part, and a gravitationally unbound Virgo supercluster, of which we are part." Does current research suggest that we are permanently bound, or only temporarily?
This more recent post (Time-frame regarding cosmic expansion and the bound state of the Virgo Cluster), for example, claims that we may be bound gravitationally to the Virgo Cluster enough to resist cosmic expansion and asks for a timeframe for which we are inexorably bound, but the answer seems insufficient to justify this assumption.


texts: Inverse of the sunrise equation - finding locations with a given sunrise time on a given day

texts:
I'm working on a project for fun where I represent some sleep data geographically. For a given day, I have a date, a time for falling asleep that night, and a time for waking up the next day. The idea is something like this xkcd comic. My attempts at taking the inverse of the sunrise/sunset equations I've found online have not been fruitful.

Basically, I would like to find an equation or algorithm to give a rough location on Earth with a specific sunset time on some specific day and a specific sunrise time the next day. I am not too concerned with accuracy since this is just for fun, so I'm hoping elevation information isn't necessary.

I'm a programmer, so the goal is really to be able to type something like

>>>> locate(date = '2014-08-27', sunset = '10:00 PM', sunrise = '7:30 AM')

and get back

Latitude: 49.887, Longitude: 96.141

I could program it myself if I just understood enough astronomy. Any help is greatly appreciated.


texts: How high were the tides back when the Moon was much closer to Earth?

texts: In this article it is stated that the Moon was originally much closer to Earth at a distance of 22,500 km rather than the 384,400 km it is today. What was the height of the tides back then in meters? Surely they would've been orders of magnitude higher?


texts:
This answer to Has great eyesight been necessary for astronomers? mentions Astronomer Jocelyn Bell Burnell's recounting of a likely first visual observation of a pulsar. This can be found for example in Nature's Air force had early warning of pulsars

  The work preceded by several months the observations made by Bell Burnell, then at the University of Cambridge, UK, which led to the first paper on the subject. A Nobel prize for the discovery was subsequently awarded to her supervisor Antony Hewish, but, controversially, not to her. Schisler was not the only one to “pre-discover” a pulsar, though, according to Bell Burnell. “There are actually a lot of stories,” she says. In the 1950s, a woman visiting the observatory at the University of Chicago, Illinois, pointed out that there was a regularly pulsating source of visible light in the Crab Nebula. Elliot Moore, an astronomer at the university, dismissed the woman's claim, telling her that all stars seem to flicker. Another radio astronomer she knows of will, after a drink or two, confess to having dismissed observations of a pulsating source as the result of faulty equipment. “He's a bit embarrassed now,” says Bell Burnell.

This is also summarized in Wikipedia's Crab Pulsar:

  Jocelyn Bell Burnell, who co-discovered the first pulsar PSR B1919+21 in 1967, relates that in the late 1950s a woman viewed the Crab Nebula source at the University of Chicago's telescope, then open to the public, and noted that it appeared to be flashing. The astronomer she spoke to, Elliot Moore, disregarded the effect as scintillation, despite the woman's protestation that as a qualified pilot she understood scintillation and this was something else. Bell Burnell notes that the 30 Hz frequency of the Crab Nebula optical pulsar is difficult for many people to see.

Per Wikipedia the Crab Pulsar has an apparent magnitude (V) of about 16.5, but I don't know how much of that intensity is modulated at 30 Hz or if the modulation is concentrated in some specific part of the visible spectrum. I would guess that a neutron star's direct visible light would be quite small except for the pulses, but I don't know if that's what the 16.5 m refers to.

I'm imagining a small, low noise photodiode intercepting the light from the region containing the location of the pulsar, possibly with a color filter, with the signal amplified and the DC removed and then digitized with an ADC and a Raspoerry Pi. (assume that I'd read up on how to best do this) and then integrating for a few minutes or hours and looking for some power near 30 Hz with a Raspberry Pi.

But before I think further, I'd need to know how deep the visible light's modulation is and if it's stronger in some wavelengths or flat across the spectrum.

Question: How bright is the Crab Pulsar's 30 Hz modulation in visible light? What color is it?


texts: gravitational time dilation multiple sources

texts:
I wondered how time would be dilated for an object in the middle of 2 black holes, however
https://en.wikipedia.org/wiki/Gravitational_time_dilation only provides a formula for one source of gravity

i tried approximating it by multiplying the time dilation factors, however when i used 1 single source with twice the mass the results differed from 2 objects with half the mass quite drastically

the examples:
mass $1,00E+036$kg distance $3000000000$m
time:$0,7105905766$

mass $2,00E+036$kg distance $3000000000$m
time:$0,099387802$

approximation: $0,7105905766*0,7105905766=0,5049389676
$
which is off by a factor of 5 and would be even more off the closer the distance gets to the schwarzschild radius

so how can the time dilation of multiple objects be approximated ?


texts: How did scientists determine an estimate of the number of planets greater than Sedna's size to exist in the Inner Oort Cloud?

texts:
In the article, New Dwarf Planet Found at Solar System's Edge, Hints at Possible Faraway 'Planet X' (Wall, 2014) where they report on research and observations performed by astronomers that discovered the dwarf planet 2012 VP113 which is smaller than, but has an orbit severely elongated like that observed for Sedna.

The article reports the claim from the astronomers who discovered 2012 VP113 that

  Based on the amount of sky the scientists searched, Trujillo and Sheppard estimate that about 900 bodies larger than Sedna may exist in this faraway realm, which the astronomers dub the inner Oort Cloud.

How was the estimate, in bold above, calculated?


texts: Why is the observable Universe larger than its age would suggest?

texts:
The age of the Universe is estimated at 13.8 billion years, and current theory states nothing can exceed the speed of light, which can lead to the incorrect conclusion that the universe can't have a radius of more than 13.8 billion light years.

Wikipedia deals with this misconception as follows:

  This reasoning would only make sense if the flat, static Minkowski spacetime conception under special relativity were correct. In the real Universe, spacetime is curved in a way that corresponds to the expansion of space, as evidenced by Hubble's law. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance. → Ned Wright, "Why the Light Travel Time Distance should not be used in Press Releases"

That doesn't clear the matter up for me, and having no science or maths background beyond high school, further reading into Hubble's law isn't helping much either.

One layman's explanation I've seen offers explanation that the Universe itself isn't bound by the same laws as things within it. That would make sense – insofar as these things can – but the above quote ("Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance") seems more general than that.

Can anyone offer (or direct me to) a good layman's explanation?


texts:
Is there a resource online of images of the moon (taken from the Earth) suitable for processing with a computer: I.e. taken with the same camera, optics, location, etc, over months/years?

Purpose: This paper looks at reconstructing the image of the Earth by looking at earthshine on the moon, but only considers a simulation. I was wondering whether it would actually work. Also I feel that there is a lot of info that the paper didn't use (and also I wonder what some machine learning techniques might achieve). One would also need historic images of the Earth (for training the system, re cloud cover).

If such a dataset doesn't exist, would it be something that could be collected? (the telescope clearly doesn't need to be very powerful, the photo exposure wouldn't take long, etc).


texts: What does the Sun "look like" below 100 MHz?

texts:
Radio2space.com's The Radio Sun says:

At very low frequencies (below 0.1 GHz) and therefore at very long wavelengths (> 3m) the solar disk appears much bigger and brighter in the center, and its brightness gradually decreases and vanishes after several solar radii.

and I guess if you sell dishes and feed horns, then < 100 MHz may seem like "very low frequencies".
This page shows an image of the Sun at 4 GHz and it's relatively dark but lined with bright "spots" in two bands on either side of the equator.
But what does the Sun "look like" below 100 MHz? Is it just a fuzzy, unresolved blob that fades to zero at several solar radii, or can one resolve the Sun's edge.
Different but related:

Do stars have "radio photospheres"? Are they different from their optical photospheres?
What will it finally take to accurately measure the distance to Betelgeuse? (includes links to radio images of other stars)

 4 GHz. (click for larger)

The radio Sun: radio image of the Sun recorded by VLA. The brightest regions are part of corona nearby but beyond sunspots. Courtesy (NRAO/AUI)


texts: Can quantum entanglement cause nucleosynthesis to happen in stars?

texts: Even if there isn't enough heat and energy in the star to cause nucleosynthesis, could atoms quantum entangle to create a new atomic nuclei? Or would it be quantum tunneling - if this is possible?


texts: Can smaller habitable planets (or moons) orbit a larger habitable planet?

texts:
I am doing some research for my fantasy novel, but I'd like to really understand how my planet system would cause life to grow if at all possible.

My idea would be, there is a large core planet that is orbiting a sun. Two smaller planets (or moons) are in an orbit around it on polar opposite sides of the core planet. Is this even possible? Would the necessary means to spark life occur?


texts: Are high-speed galactic collisions survivable?

texts:
This is a hypothetical question, but it really is meant as a vivid way of asking about the affects of high-speed galactic collisions. The November 27 APOD showed the Cheshire Cat galaxy group, and the caption says that the "eyes" are two large elliptical galaxies, closing on one another at over 1000 km/s, and bathed in a glow of X-rays from heating of the associated interstellar media.

How intense is this glow? I suspect it is very faint, but I don't know how to quantify it. Would it hinder X-ray astronomy for residents of the galaxies? Would it be strong enough to affect life? Does anybody have quantitative information and know how to interpret it? (I do realize that elliptical galaxies are thought to be made up of old, low-metallicity stars that aren't good candidates for habitable planets, but let's ignore that.)


texts: Can the asteroid belt be seen from Jupiter?

texts: Suppose that someone lives in the neighborhood of Jupiter. He may live in a space station around Jupiter, or in any ot it's moons (I believe that it doesn't matter for this question). If this person look to the Sun he can see the asteroid belt, since it is between them? If generally not, then he would be able to see larger bodies such as Ceres?


texts:
What do we know about extraterrestrial snow? On which (exo)planets or (exo)moons do we have direct hints for its existance? This is indeed a children's question, but I struggle to answer it conclusively.
I only know about snow on Mars, Venus snow (which strictly speaking might be a misnomer) and snow on Titan.
For exoplanets in the habitable zone, the phase diagram of water may of course allow snow if water existed on an exoplanet. What about non-water snow? Which chemical substances which allow snow have been observed so far on other (exo)planets?
Generalized definition of snow
As snow I would define as precipitation of any (chemical) kind which is not in liquid or gaseous state. The solid objects (most likely crystals) should be moving downward in from the sky and is possibly reaching the ground (if there is any).
This definition excludes ground frost and hail. Hail is excluded since these layered balls of (water ice and/or other solids) are actually moving repeatedly and significantly moving upward (on Earth in Comulonimbus clouds). The definition does include solid clear sky precipitation aka diamond dust.


texts: Particle horizon distance as $t \to \infty$

texts: So I know that the particle horizon distance in our universe is about 46 billion light years. I also know that as $t \to \infty$, the particle horizon distance approaches a finite value. What is this value and how can it be calculated?


texts: Does Einstein's theory of gravity say anything about the direction of space

texts:
Why is every representation of Einstein's theory of gravity depicted like this:

and not like this:

And many other ways it can be depicted. They suggest that the fabric of space-time is a flat plane


texts: Is it possible that quasars are not powered by black holes but some other kind of compact, massive object?

texts: AFAIK, objects in the universe thought to be black holes show evidence of radiatively inefficient accretion flow. But is this the case with Quasars? If not, is there a possibility that the core of a quasar is an object similar to a neutron star.


texts: Are X-ray telescopes with glancing angle surfaces basically "funny-looking" Cassegrain telescopes mathematically?

texts:
In this answer I included the image below of a reflective X-ray telescope. It is made from two elements; the first is concentric shells of glancing (high incidence) angle paraboloidal surfaces, and the second element is the same except that they are hyperboloidal surfaces.
A "Classic" Cassegrain telescope is also a parabolic mirror followed by a hyperbolic mirror, except that the configuration looks very different since for visible light we can use near-normal incidence and are not restricted to high incidence angles.
Question: Despite looking so different, are these basically the same thing? Is the mathematical relationship between the parabolas and the hyperbolas the same in both cases?
Related in Space SE: How does this strangely-shaped horn at Honeysuckle Creek Tracking Station work?

Source


texts: Calculate telescope orientation based on RA, DEC and Lat/Long

texts:
I'm parsing FITS files for a project based on data from a telescope. These files include 'DEC', 'RA' and lat long values. I understand roughly the concept of celestial coordinates and I assume that these values are sufficient to calculate the direction that the telescope is facing and the angle that it's at but my trigonometry is not up to the task.

I am using astropy for the project so any answers which reference that would be great.


texts:
When there are accretion discs jets of particles or radiations will be emitted in the direction parallel to the rotational axis.

My beginner level astronomy textbook just says that it is due to magnetic field but does not explain why.

So can anyone explain the mechanism in layman's terms? Thanks in advance!


texts: Why isn't Ultima Thule spherical, while its two component parts are

texts: The two component parts of the Ultima Thule contact binary are approximately spherical. Why have the same forces that made the two parts spherical when they were separate not made the whole thing spherical after they joined?


texts: How thick are the intergalactic filaments?

texts:
Recently, in October, news came out about the faint intergalactic filaments finally being 'found'.
Then, in June, astronomers announced that these things had enough normal matter in them to solve the 'missing baryon problem'.
Although their length(s) are often mentioned, nowhere do I hear about their width/thickness....
These 'Warm-hot intergalactic filaments' are described as long, thin filaments of plasma;  brought together by gravity....
Does anybody have an idea as to these filaments' thickness/width?


texts: How to determine atomic number density of an element in a star based on equivalent width measurements

texts:
Given an equivalent width measurement $W$ of a spectral line of element $X$ and the effective temperature $T_{eff}$ of a star, how can you determine the atomic number density of $X$ in that star?

According to Warner 1965, for the 6707.8 Lithium line we can apply the approximation

$$\log{\frac{N^*(X)}{N^{\odot}(X)}} = \log{\frac{W^*(X)}{W^{\odot}(X)}} - 4.93 (\gamma -1)$$

where $\gamma = \frac{T^{\odot}}{T^*}$ and $\log{W^{\odot}(X)}$ must be measured. The same paper says that $\log{W^{\odot}}(Li)=-6.15$ (my goal is to solve for the abundance of Lithium given a measured W of the 6707.8 $\overset{\circ}A$ line.

Under what regime is this approximation valid? Also, I am unable to solve this equation because I don't know what $N^{\odot}(Li)$ is. How can I determine a reputable value for $N^{\odot}(Li)$?


texts:
The pattern of stars on the surface of the celestial sphere seems perceptually to have an universally looking structure. If you pick a region of the sky at random, for instance by using the (excellent) set of tools from the stellarium suite you get for example this:

There are bigger structures, like the milky way or more clustered areas, but on a smaller scale (like that you will see in binoculars with a field of view of the size of the moon), you will very often get this similar pattern. This looks to me very much like a self-similar fractal and I am curious to know if anybody knows more about it.
Assuming a given density of stars in this patch of the universe, and  omitting inhomogeneities at first,  can we derive a mathematical expression for describing the density of stars in a given patch of the sky ?
As a bonus, is there a map estimating this density for different areas of the celestial sphere? Clearly, the milky way is more crowded and would perhaps have more structure...


texts: Why does Titan have lower surface gravity than the Moon when Titan is more massive?

texts: Reading on Wikipedia I saw that Titan is 80% more massive than the earth's moon but has only 85% the surface gravity. Why is this?


texts: What would be the sizes of the bodies created from Jupiter's trojans?

texts: If all trojans from the Greek camp and all the trojans of the Trojan camp were combined into two bodies respectively, what sizes and masses these bodies would have? Would they qualify as dwarf planets? Would they be greater than Ceres?


texts: About more than 15 stars moving in straight line through the sky

texts: I had seen more than 15 stars are moved in a straight line through the sky just like a train. And the distance between each star was not same, some of them were near and some of them were far. The intensity of the light of each star was also different. I had seen this phenomena on 21st November 2019 at Indian time 5:30 AM in my place. What could that have been?


texts: Has this image of the Draco Constellation been altered

texts:
Have the stars in this photo of the Draco Constellation been magnified so as to help me see the Draco or not?

Also I really like [this image][2] of Orion; if someone knows of a site with a lot of such images that are in a user friendly, easy-to-find format, I would appreciate it.


texts: Precession and the location of the sun on the horizon at equinox

texts: The sun lines up with this fountain, which is at the Salk Institute, during the equinoxes. Will precession eventually throw the alignment out of whack?


texts: Is the universe moving through infinite space time as it expands?

texts: What I mean by this, is imagine a person blowing up a balloon. As the balloon expands, the person blowing it up is also walking. At a much grander scale, apply this concept to the universe. As the universe expands, is it also moving like the balloon through infinite space time?


texts: Why does the darkening of the Moon go from left-to-right during an eclipse?

texts:
In anticipation of the upcoming April 2014 lunar eclipse, I decided to use the 3D virtual planetarium software Stellarium to simulate what it is supposed look like where I live, since I've never actually witnessed an actual eclipse before. I set the clock to begin at the moonrise before the beginning of the event and to tick at a rate of a few virtual-minutes per real-second, pressed play, and waited for the eclipse to begin. When it did, I saw something I did not expect. To be honest, I realized my expectations were embarrassingly naïve.

I watched the Moon rise in the east and begin its upward left-to-right path across the night sky, towards its zenith in the south. My expectation was that the moment the Moon enters Earth's shadow which marks the beginning of the eclipse would be visually apparent by the darkening of the full moon's outer limb on the right side, and the darkening would progress from right to left from there. When I saw that the darkening actually progresses in the opposite direction, I immediately realized that my understanding of the geometry and kinematics of the Earth-Moon-Sun system must be much shallower than I would have thought.

My main question is must lunar eclipses happen this way every time, and if so why? My gut is now telling me that it does, so I'm mostly interested in an answer of reasonable depth to the 'why' part of the question.

Additionally, it occurs to me that the observation of left-to-right progression of darkening of the Moon's face during a lunar eclipse is especially counter-intuitive from the point of views of ancient geocentric cosmologies based on a stationary Earth that doesn't rotate. Can this observation be taken as empirical evidence that the Earth rotates? (I could be completely off base here. If so, please correct me.)


texts:
Recently NASA has revealed that they have recorded the sound of Sun. They say that it produces a sound like "Om". I can't understand how they can hear it.

To the best of my knowledge, sound can't travel in space (vacuum). Has a new technique been invented that enables them to detect sound coming from outer space? If so, what is it? How does it work?


texts: How much of the sun needs to be blocked in order to see a noticeable drop in brightness

texts:
I live in NYC and just experienced the eclipse. What fascinated me the most was that despite the fact that the sun was about 70% blocked, the area was not noticeably dimmer. If the news wasn't saying there was a solar eclipse I wouldn't have realized it was happening.

How much of the sun needs to be blocked by the moon in order to noticeably dim an area?


texts: Is it possible to do moon sighting in advance for 5 years with 100% accuracy?

texts:
It is Ramzan and the usual heated disagreements between clerics here in Pakistan as to the sighting of the crescent. This time the minister of Science and Technology (who is not a scientist himself btw) has intervened and formed a committee comprising of meteorologists and our space agency. This committee is tasked to form a 5 year lunar calendar, specially one that will fix the issue of moon sighting of major religious months and events such a Ramzan, Hajj (pilgrimage), Eids etc.

My question is that: Is it scientifically possible to predict (simulate...) in future with 100% accuracy as to the location of the moon? My surface reading of Lyapunov time suggests that the solar system can be predicted with accuracy for 5 million years.

I would like to know your understand on this matter.

Also some commenters mentioned that this question might be a duplicate of an other one. I would like to add that there are school of thoughts and in one the condition is that the crescent is to be observed from the surface of the Earth. So I think this question is unique in that sense that it includes that component too.


texts: How (the heck) was this photo of Venus at inferior conjunction (between us and the Sun) taken?

texts:
This great answer by @gerrit discussing planetary phases seen in visible light contains the image I've included below. According to the Wikimedia Commons link these are ESO images from the Venus transit 2004 VT-2004 observing campaign.

The ESO, or European Southern Observatory:

  Organisation Européenne pour des Recherches Astronomiques dans l'Hémisphere Austral) is a 16-nation intergovernmental research organization for ground-based astronomy. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky. (emphasis added)

These images are very high quality, and the last one, dated 8/6/04 (08-June-2004) appears to be taken at/near inferior conjunction, when Venus is almost directly between the observer and the Sun.

Question: I'd like to understand with what instrument this image was taken, and how the technical challenges imaging Venus with such a small angular separation between it and the Sun were overcome! I did a quick check and during the UTC day of June 8th, 2004, Venus moved from about 0.6 to 1.1 degrees from the Sun as viewed by Earth. How was Venus imaged so nicely? Was this really imaged from the ground through Earth's daytime atmosphere, or from space? In either case, how was a high quality telescope pointed so close to the Sun and shielded from stray light and protected from damage?

below: Phases of Venus, from here.


texts: Is the time lapse considered when estimating the age of the universe?

texts:
We say that our universe is 13.7 billion years ago. During the big bang, it doubled at least 90 times in trillionth of a second (as given here), and other topological statements.

The question is which time are we talking about. Is the time-lapse considered? If yes, how?


texts: In galaxy optical spectra, why are Hbeta and Hdelta sometimes absorption lines?

texts:
I've heard that $\mathrm{H\beta}$ and $\mathrm{H\delta}$ lines in galaxy spectra might be either emission lines or absorption lines, is that correct?

From Wikipedia, Balmer lines are usually emission lines in the spectra of spiral and irregular galaxies probably due to the presence of type A stars. If ellipticals lack hot (O, B and A type?) stars, and are gas poor(?), then is it the remaining stars doing the absorbing?

Also, are the CaII-H and -K doublet ever emission lines?


texts: Pivot scale in cosmology, CMB observatiion

texts:
What is a pivot scale, pivot frequency in general, and particularly in the cosmology? I don't get the idea.
Almost everywhere I found the same the same sentence "we take the standard value of a pivot scale $k=0.05$ Mpc$^{-1}$ for Planck and pivot frequency as $f_{cmb}=(c/2\pi)k$". What does it mean?
(For example in https://www.cosmos.esa.int/documents/387566/387653/Planck_2018_results_L10.pdf)


texts: Is it possible to create an artificial Black Hole?

texts: I am quite curious about black holes. Once I just thought whether it is possible to create an artificial Black Hole, i.e. an human made black hole. Also explain there formation if it is possible.


texts: Could the barycenter orbit of our sun be greatly underestimated?

texts: The radius of the suns orbit around the barycenter of the solar system is often measured to be roughly the radius of the sun. Based on the same type of bias that has made humans miss things about their solar system because they are looking at it from the vantage point of Earth orbiting inside the system, is it possible that the suns orbit around the barycenter is actually much larger?


texts: Where might a semi proficient amateur analyst participate in meaningful astronomical efforts

texts:
I am a retired engineer that has an ongoing interest in space efforts.  In my youth I did work on the Apollo program but on propulsion and vehicle thermal control: not flight dynamics.

I have reviewed/studied some astrodynamics (Vallado, Bates, etc) and can run GMAT and tudat libraries from Delft University.  I have explored Systemic Console, tap, and other software for implying the existence of exoplanets from RV curves and light transit curves.  I can download data in FITS format from various astronomical databases.

Where might I direct my efforts in some meaningful way by participating in a project, collaborating with an investigator, or pursuing some research oriented endeavour on my own?   Could someone point me in a direction where I might do some meaningful work?  Just picking a star and starting a transit analysis does not seem to have any probability of leading anywhere.

Those sites where one views a light transit curve or tries to classify a galaxy type are not very interesting to me.  I feel I could investigate things more in depth than that.

Directions, Ideas, offers! Anyone.
I will post this in Space Exploration forum also.
Thanks,

Tom Kosvic


texts: Location of the milky way in August in Death Valley

texts:
I am going to be in Death Valley around August 8th. I was hoping to take a milky way shot with my camera when there. I would be shooting around 10pm.

I am using google maps to scout out popular viewpoints in Death Valley for potential locations for the shot, as I will be getting terrestrial objects in the frame.

From what I have read , I believe the milky way will be visible most of the night, and best viewable in the earlier hours of the night. In addition it will be in the south sky.

So will it be precisely due south? Or will it be slightly south west or south east? If I could pinpoint the location, it would be easier to scout out a spot.

Thanks!


texts:
I'm prototyping a real time solar system / milky way discovery app.

Could you please help me on:
-What is the best orbital data public API I can use?
-What is the standard numerical way to define orbit?

I'm looking for something like JPL Small-Body Database Browser, but for every celestial objcets, and / or missions.


texts: Do narrow lines in the spectra of O- and B- type stars always indicate magnetic fields?

texts:
I was reading a paper on the differential emission measures of a set of hot O- and B- type stars. As the authors discuss in Section 3 (page 959), two stars, $\tau$ Sco and $\theta^1$ Ori C, have narrow emission lines compared to other stars of the same spectral type. There is a good amount of evidence that both stars are young and have magnetic fields; the stellar winds travel along the field lines and collide at the magnetic equator, producing shocks and, indirectly, X-ray emission. This is in contrast to the mechanism behind shock formation in other O- and B- type stars, where shocks come from the line instability transition.

Now, the plasma at the magnetic equator is roughly stationary because of the collision; this means - if I'm interpreting things correctly - that there is little to no broadening of the lines, and so they are comparatively narrow. Other spectral characteristics and observations support this model.

I'm wondering two things:

Has the presence of such comparatively narrow lines in the spectra of O- and B- type stars been observed in cases where there is no magnetic field present? Very few O-type stars have significant magnetic fields, and theory predicts that none should, so these stars are the few exceptions to the rule.
If so, are there other mechanisms that could be responsible for this in cases without magnetic fields? I'm grasping at straws for ideas, like low collisional broadening.

I talked with my advisor, and we briefly discussed colliding wind shocks (CWS), which form at the interface between the stellar winds in a binary system. It's thought that these shocks contribute only a small amount to the total X-ray production of the system (see Gagné et al. (2011)), though, so any narrow lines produced in that plasma - if any exist - wouldn't have much influence over the measured spectra. So that seems to be ruled out.


texts: Why does time pass faster in moon than the earth?

texts:
One of my friends said that,

time passes faster in moon than the earth

Why does time pass faster on the moon than on the earth?


texts: Any book on `How do they know it (in Astronomy)`?

texts: I am reading my college level undergraduate astronomy textbook. It has lot of ínfromation but every page rises a similar question in my mind: "How do they know it?". It starts from simple things like radius of earth, other planets, masses, composition, distances of stars, temperatures, luminosities etc. How did scientists know these is a question that pops up in my mind about every fact I read. Do you have any book recommendations which mostly (if not exclusively) focuses on the question "How do scientists know it?"


texts:
I've heard that KIC 9832227 is a contact binary star system that is expected to merge and explode as a "red nova" sometime in the next ~5 years or so.

http://www.sciencemag.org/news/2017/01/colliding-stars-will-light-night-sky-2022

Before that happens, I'd like to see it.  Where would I need to be / what sort of equipment would I need in order to be able to view this before it goes nova?


texts: Is the acceleration of the expansion of the universe a direct observation or an interpretation?

texts:
Is the acceleration of expansion we observe with supernovae direct or is this an interpretation according $\Lambda$CDM model ?

I mean what we observe is luminosity distance versus redshift, so to see acceleration we should have scale factor vs time, which need a model, right (assuming Friendmann-Lemaître equations in which we need to consider homogenous universe for example) ?

Is it possible to say "our universe is in accelerated expansion" without any  interpretation with a model ?


texts: Is there evidence for the speed of gravity?

texts: Is it an assumption that the speed of propagation of gravitational waves is exactly the same as the speed of light? If the speed of gravity is only slightly less than the speed of light, the effect might not show up over solar or intra galactic distances, but over extragalactic distances could it give the appearance that there is dark matter influencing motion?


texts: How can I find a date when the moon at zenith over a specific spot on the equator?

texts: I would like to know when the moon is directly overhead 0° S, 166° E. Are there any websites that will show me any of those dates?


texts: Determining latitude and longitude in bad weather

texts:
In Sobel's famous book 'Longitude', she described the story of Harrison. I still have one question. What if it is cloudy on the sea? In this case, the sailors have nothing to compare with, right? This still means in many many cases, they have no means to determine their position, neither latitude nor longitude, right?

It seems that the problem is not so satisfactorily solved.


texts: Need help on Springel model for galaxy simulation

texts:
I am working on a code for galactic dynamics simulation. Initial conditions (positions and velocities) are based on a model with a massive dark halo and a thin exponential disk.

Now, from the following paper "Tidal tails in CDM cosmologies", I am studying the response of the dark matter profile.

Here's a capture of the article part that confuses me :

Firstly, I don't understand how one gets equation (16) from the fact that angular momentum of individual dark matter orbits is
conserved, i.e :

\begin{equation}
r_{i}\,M(r_{i})=r_{f}\,M_{f}(r_{f})\quad\quad\text{(eq 16)}
\end{equation}

" where $r_{i}$ and $r_{f}$ are the initial and final radii of some dark matter mass shell, $M(r)$ gives the initial NFW mass
profile, and $M_{f}(r)$ is the final cumulative mass profile after the disk is formed."

Question 1) Is (equation 16) got by the momentum theorem ? ($\dfrac{\text{d}\vec{L}}{\text{d}t} = \vec{OM} \times \vec{F}$)

And how to prove it, i.e what's the demonstration to get (equation 16) ?

I don't understand what's the difference between $M(r)$ and $M_{f}(r)$.

After, it is said that $M_{f}(r)$ is the sum of the cumulative mass of the disk and the dark mass inside the initial radius
and author writes (equation 17 on paper) :

\begin{equation}
M_{f}\,(r_{f})=M_{d}\,(r_{f})+(1 - m_{d})\,M(r_{i})\quad\quad\text{(eq 17)}
\end{equation}

I would like to connect this equation with the total mass of galaxy which could be written (according to me) as :

\begin{equation}
M_{total}\,(r) = M_{d}\,(r) + M_{DM}\,(r) = m_{d}\, M_{DM}\,(r) + M_{DM}\,(r) = (1+m_d)\, M_{DM}\,(r)
\end{equation}

with $m_{d}$ the ratio between disk mass and halo mass.

Question 2) Could you help me to connect the total mass with this equation (17) ?

After with equation (18), author of paper writes that the final profile $M_{h}(r)$ of the dark matter halo is then given by :

\begin{equation}
M_{h}\,(r) = M_{f}\,(r) - M_{d}\,(r)
\end{equation}

Question 3) So should I conclude that $M_{f}\,(r)$ is the total mass of galaxy (disk + halo) as a function of $r$ ?

Any help would be great,

Regards

UPDATE 1 :

I have contacted the author of paper "Tidal tails in CDM cosmologies", i.e Volker Springel and here are its comments :

  Question 1*) Answer: For simplicity, you can consider that all particles are on circular
  orbits. The initial circular velocity of a particle is $$v_i = \sqrt{(G M(r_i)/r_i)}$$, the final one is $$v_f = \sqrt{(G M(r_f)/r_f)}$$. The
  corresponding angular momenta are $$L_i = m r_i v_i$$ and $$L_f = m r_f v_f$$.
  Now, the angular momentum is an adiabatic invariant when the halo is
  slowly compressed, hence $$L_i = L_f$$. The latter equation then gives eqn
  (16) when one plugs in the velocities.

  Question 2*) "I don't understand what's the difference between $M(r)$ and
  $M_f(r)$ ?"

  Answer:

  $M_i(r)$ is the initial and $M_f(r)$ the final cumulative mass profile.

  Question 3*)

  I would like to connect this equation with the total mass of galaxy which could be written as :

  $$ M_{total}(r) = M_d(r) + M_DM(r) = m_d M_DM(r) + M_DM(r) = (1+m_d) M_DM(r)$$

  Answer : This can only be written for the initial distribution in
  this form, where the assumption is that the mass that will later make
  up the disk is distributed proportionally to the dark matter.

  Question 4*)
  Could you help me to connect the total mass with this equation (17) ?

  After with equation (18), you write that the final profile M_{h}(r)$ of the dark matter halo is then given by :

  $$M_h (r) = M_f(r) - M_d(r)$$.

  So should I conclude that M_f(r) is the total mass of galaxy (disk + halo) as a function of r ?
  < Answer : "Yes."

In conclusion, I can solve my question 1*) but not question 2*) nor question 3*)

Right now, my goal is to demonstrate how to get  :

$$\begin{equation}
M_{f}\,(r_{f})=M_{d}\,(r_{f})+(1 - m_{d})\,M(r_{i})\quad\quad\text{(eq 17)}
\end{equation}$$

Any help to acheive this is welcome, thanks


texts: Any good books to read for complete beginners?

texts:
Currently, I am a Ph.D. student(mathematics) but always loved the field of astronomy hence I would like to read something on the "side" in this field along with my work. So is there any good introductory book(s) for me to read?

Thanks, any help/guidance will be greatly appreciated

Edit: I, personally, wouldnt call it 'too broad' as the question is asking for some introductory books but if not then, its fine :)


texts: Deriving Dark Matter; specifically looking for a table of stellar speed versus distance from center of galaxy to derive dark matter

texts: I'm trying to find some data that I can use to derive dark matter (in a loosey-goosey sort of way, I won't be too rigorous). I'm helping a friend out with a final project for his astronomy course and I'm tasked with doing the maths. Any idea where I can find a table of various stars distance from the center of our galaxy and their speed (how fast they are going around the center of the galaxy)? This does not need to be comprehensive but would ideally contain several data sources from various representative radii. Also some sort of formatted text would be SUPER cool so I could just tell my computer to do the calculations!


texts:
After I back from Minimart, it's about 6pm though, I saw the sky, yeah a beauty evening sky, but I sense a something weird from the sky, It look like a star at first it moving in low speed, I Though that was a Kite or plane, but it cant be plane cuz it moved up then turned right then turned left and get back.
it size and it light same as a normal Stars.
It's move like as Kite, but it so bright like star.
after a few minutes, it's stoped move, but then its Moved Fastest Like A shooting stars, and yes it has a tail like Comet,that moved in Light speed.

after that I got back to home,
I asked to my friend on FB,
I said that I saw a moving star,
I didnt say it when,
but my friend asked me back,
"was that do along 6pm?"
"whoa, how can be You know that?"
and then my friend said if that was an UFO
he ever saw that many times.

(sorry for bad English)


texts: Is there any way through which we could determine the ROTATION period of an exoplanet based on the data we observed?(from transit)


texts:
How fast is a ringularity1 spinning? Spinning black holes are created from spinning stars.
The angular momentum of the 'parent' star must be conserved so spinning black holes are spinning much much faster than their 'parent' star.
Since black holes have generally less mass than their parent star and mass is found only in a tiny amount of space then the ringularity should be spinning incredibly fast.
The ringularity is a ring which contains all the mass of the black hole and it is in the center and it is almost infinitely tiny so it must be spinning almost infinitely fast.
Can it spin faster than c? Is there any way to know if SR can be violated inside such extremely dense objects?
1Ring singularity


texts: Is it possible to view the Andromeda galaxy with the naked eye in Greece?

texts: I am living in the northern hemisphere in Greece and in class 4 area (light pollution scale). Is it possible to see the Andromeda galaxy with a naked eye?


texts: can we calculate the velocity of galaxies at the edge of the observable universe?

texts:
I read this post

Does the edge of the Universe travel faster than the speed of light?

but there was no calculation, so I tried it thusly:

The observable universe is 28.5 Gpc (gigaparsecs)
= 28500 Mpc (megaparsec)

https://www.google.com/search?q=diameter+of+the+observable+universe&oq=diameter+of+the+observable+universe&aqs=chrome..69i57.6887j0j7&sourceid=chrome&ie=UTF-8

The Hubble constant is 67.4 or 70 km/s/megaparsec.

https://www.nasa.gov/feature/goddard/2019/new-hubble-constant-measurement-adds-to-mystery-of-universe-s-expansion-rate

I’ll use the older value 67.4

v=H x D = 28500 Mpc * 67.4 km/s Mpc = 1,920,000 km/s = 1.9 x 10^9 m/s

speed of light 2.998 x 10^8 m/s

So the velocity of expansion at the edge of the universe is >6x greater than the speed of light?

I read these posts

https://physics.stackexchange.com/questions/107748/how-are-galaxies-receding-faster-than-light-visible-to-observers

So where are these measurements of galaxies moving faster than light?

and this

https://arxiv.org/pdf/astro-ph/0011070v2.pdf

From the arxiv paper, I get that galaxies can be receding at 3c based on GR, but my simple calculation comes up with 6c.

Can someone help me reconcile my curiosity with the facts?


texts: What is the thickness of the Solar System disk?

texts:
What is the thickness in AU of the all planets' orbital plane height combined in the Solar System?
Excluding Pluto.
Looking for h


texts: Do supernovae produce powerful gravitational waves?

texts: I would think that the amount of energy produced by one supernova is much more than a pair of colliding black holes, no? Wouldn't that much material and energy output from the explosion generate a powerful gravitational wave?


texts: Where is the exomoon? (Possible discovery of a companion to Kepler-1625b)

texts:
The new paper in Science Advances Evidence for a large exomoon orbiting Kepler-1625b (open access!) uses a sophisticated combination of occultation light curve simulation, signal processing and statistical analysis to show that there is a significant chance that they have identified an exomoon, a moon orbiting an exoplanet.

But it's too sophisticated for me to understand how this Hubble data indicates a potential moon. Terms like "detrending",  "Markov chain Monte Carlo", and "Bayesian evidences" are beyond me.

Is it possible to choose one of those plots and add an arrow to say "this blip here, this is Kepler-1625b potential moon"? If not, is it possible to at least explain in a simple way what it is from this analysis that has led them to believe there may be a moon there?

Abstract:

  Exomoons are the natural satellites of planets orbiting stars outside our solar system, of which there are currently no confirmed examples. We present new observations of a candidate exomoon associated with Kepler-1625b using the Hubble Space Telescope to validate or refute the moon’s presence. We find evidence in favor of the moon hypothesis, based on timing deviations and a flux decrement from the star consistent with a large transiting exomoon. Self-consistent photodynamical modeling suggests that the planet is likely several Jupiter masses, while the exomoon has a mass and radius similar to Neptune. Since our inference is dominated by a single but highly precise Hubble epoch, we advocate for future monitoring of the system to check model predictions and confirm repetition of the moon-like signal.


texts: In 2016, the summer solstice will coincide with a full moon. How often does this happen?

texts: I found that the solstices work on a 400 year cycle based on this. I can't find anything similar for the lunar cycle. Is there a formula I can use to calculate the date of the full moon?


texts: Could a star closely orbit a black hole long enough for the star to have lost 0.5B+ years to time dilation?

texts:
I was wondering how stable a close star-black hole system could plausibly be, and thus how much time a star could plausibly miss out on (from an outside observer's perspective) due to being in an orbit at relativistic speeds around a black hole.

E.g. could there plausibly be a scenario where a star has only aged less than 13 billion years, but is in fact one of the universes's earliest Population II stars, because it has been in a fairly stable, close, rapid orbit around a black hole for the past few billion years*, and thus was subject to time dilation during a few billion years of orbits?

(*Or was orbiting a BH at some point, and escaped due to a chance interaction with a newcomer third body that tossed it out.)

Perhaps this would require a larger black hole, so that the tidal forces would be lower, which would presumably causing less gas stripping for an orbit that would result in the same time dilation? I realize that even well outside the photon sphere of a black hole, there can be decay of orbits due to gravity waves, but evidently there can be a counteracting separating phenomenon well, due to stars being stripped of some mass (e.g. as described in this report of a star orbiting a black hole at a velocity of about %1 of c).

So to put a number on this to make it more answerable, could a star plausibly orbit a black hole long enough and closely enough without spiralling in (or outward due to gas stripping) to make it half a billion years older in origin (according to the CMB rest frame) than it has experienced?


texts: Eagle Nebula (Pillar of Creation) destroyed?

texts:
How do they know that the pillars have been destroyed if the light will reach us in 1000 years time?

  The "Pillars of Creation" from the Eagle Nebula. Evidence from the
  Spitzer Telescope suggests that the pillars may already have been
  destroyed by a supernova explosion, but the light showing us the
  destruction will not reach the Earth for another millennium.

Article explaining that we won't see the effects in another 1000 years


texts: Can I look at the sky and find the day of the week?

texts:
Suppose I wake up from a coma on a desert island in the 19th century (i.e. we already use the Gregorian calendar but have no satellites yet). I have a clear view of the sky and a couple of days to make observations and calculations. Can I determine the current day of the week? Also, can I find the answer instantly?

On a related note, is it possible to define days of the week in a purely astronomical (e.g. planet-agnostic) manner such that for the Earth we get the traditional meaning?


texts: Are there any Stars we know don't have planets?

texts:
We have been looking for planets around other stars for a while (see related https://space.stackexchange.com/questions/1484 ) and we are finding lots of planets, some of them are Earth like According to an the answer at https://space.stackexchange.com/questions/4062 there are over 14,000 stars within 100 light years of Earth, the answer goes on to say there are "1,500 potentially habitable planets within 100 light-years" of Earth

Based on current trends it looks like there are lots of stars with lots of planets.  I am beginning to assume that our solar system is not unique and that  every star has several planets.

Are there any Stars that we know are without any planets?


texts: Caves traced in nine planets of solar system

texts:
Are there chances of caves being found or traced on other planets of the solar system?
What's the solution?

Manufacturing special telescopes for viewing or observing caves on other planets?

or

Satellites revolving around Mars, Venus & Mercury sending images back to planet Earth for assistance?


texts: Why the continued obsession in measuring the year as a multiple of days?

texts:
Pick a any notion of year and any type of day measurement.  Outside of history and the common cycle of day an night, why are these viewed as integrally or even fractionally commensurate?

Why all the stunts with leap days and leap seconds, etc... when we now have atomic clocks?  Why not decouple the two?


texts: Can Astronauts Aboard ISS See the Shadow of The Moon on The Earth During an Eclipse?

texts: I was wondering if the astronauts on the ISS are able to see the shadow from the moon on the earth at all during the eclipse.


texts: Why do astronomers use supernova to measure distance in space?

texts: Why can't the distances between our Sun and the celestial objects be measured directly and instead had to rely on looking around for supernova? Is these events frequent, evenly distributed and lasting enough for astronomers to measure any place in our observable universe? Is this method of measurement more preferable than others currently?does this method had any limitations such as an absent of supernova nearby?


texts: How to find Greenwich Mean Sideral Time?

texts:
I'm writing a program that converts position in the ECI (CIS, EPOCH JD2000.0) coordinate system to WGS 84 (CTS, ECEF) coordinates. To do so, I'm following what's described in this document and implementing C++ code.

My knowledge of astrophysics is very limited, so I find it hard to understand some terms. Especially, I can't seem to understand properly how the Greenwich Mean Sideral Time is computed, and why I always get different values.

For instance (assuming time is 2019-01-01 08:00:00 and Julian Date is 2458484.833333):

If I use this on-line GMST calculator, I find that GMST is 14:42:45.3766.
If I use the expression used in this MATLAB script (showing the value in time, not degrees) I also get 14:42:45.350.
However, if I calculate it myself with the expression given in the above appendix, I get 06:42:45.379163 (with a clear offset of

According to the equations in page A-26, GMST depends upon the number of Julian centuries (Tu). H0 expression is exactly the same than that of CelesTrak for θ(0h). However, I can't really understand what the du truly stands for, or more specifically, why the document shows du → ±0.5, 1.5, 2.5.... Moreover, to find Λ (which is stated to be in seconds of time) the expressions use values in radians and seconds of time indistinctly.

Could somebody shed light into this?


texts: How does time "add up" for observers outside of and travelers inside of a Krasnikov tube?

texts:
A ship travels 99% of c from Earth (leaving at Time A, say 2016) to Destination Star (arriving at Time B, say 2020, accounting for relativistic effects) to stretch the tube, leaves a buoy at Destination Star, then uses the tube's topology to return to Earth, effectively faster-than-light, moments after it left, according to how the Krasnikov tube is supposed to work (Time A plus a few minutes). Can the ship now return to Time B (2020) from "Time A (2016) plus a few minutes" using the tube just as quickly? If so, would its return to Time B mean "Destination Star in the ship's year 2020, the moment when it arrived / first stretched the tube to this destination?" And if so, would it mean returning to the system "before" the ship dropped the buoy?
My understanding of a Krasnikov tube is that it's a theoretical means for FTL travel that doesn't violate causality, because a ship traveling along the return path of a tube "rolls back" time to just moments before the ship originally left.

Now, I understand that there is a way to construct a return tube that would violate causality (http://arxiv.org/pdf/gr-qc/9702049.pdf), but let's assume for a moment there's some "chronology protection" in place that doesn't allow that to happen (the tube implodes when you try to do that), and that the question of the exotic matter needed to create such a tube isn't an issue for the ship's makers.
The questions I have then are:

A) Am I even thinking about this correctly? Where have I messed up?

B) I don't understand how a system of Krasnikov tubes, as some sources suggest, could be used to create an interstellar network for FTL travel. It seems to me that the traveler in the ship can never actually have an impact on his exit or return points, since he will always be returning to Time A (2016) moments after he left, or Time B (2020), the moment he arrives?


texts: If wormholes exist and behave like we think they do, can we cross them without "negatively charged matter "?

texts: I heard and read that a wormhole will collapse if they transport any positively charged matter. So, if wormholes exist, they probably close as soon as normal matter enters it, how can we access them without negative matter?


texts: Why are only the poles of Mars frozen, despite too cold temperatures on lower latitudes?

texts:
Actually the entire red planet is very cold, cold enough for ice and frost to form not only on the poles. Temperature rises above 0 deg Celsius in summer only. Why then are just the poles frozen, and not lower latitudes during most of the Martian year? Is it because most of these areas perhaps aren't within the triple point of water, unlike the poles?
Related questions on StackExchange

Mars' atmosphere and the triple point of water
Which “exotic salt” can lower water's freezing point by 70 °C?
Images of Mars Phoenix Lander showing what's happened after more cycles of winter ice build-up?
What's the scientific evidence of water for return trip methalox on Mars?
Would snow fall on the surface of Mars because of uncovering the ice deposit in Utopia Planitia?
What is the warmest place on Mars?


texts: Change In Direction of Asteroid Approaching Sun

texts: A perfectly spherical asteroid on a straight line trajectory with no rotational movement at all, enters the gravitational field of the sun (from elsewhere far far away). When it exits on its hyperbolic path, does it still point in the same direction it entered, or does it rotate to face the direction of travel?


texts: How often do stars pass close (~1ly) to the Sun?

texts:
Interstellar exchange of massive objects is difficult across several light years. But as the stars orbit the galaxy the distances between them change. I don't find data for neighbor star distances which covers more than a couple of a hundred thousand years. I can imagine that it is difficult to trace their orbits further given todays uncertainties in distance measurements. But I'm interested in a purely statistical/geometrical estimation of how frequent nearby Sun-star passages have been.

How many times has a star come closer than, for example, 1 light year from the Sun since it formed?
How well mixed have the nearest stars become over time? What fraction of them have followed the Sun since formation and made multiple passages?

I suppose that 1 light year's distance is enough to cause disturbances in the Oort cloud and likely exhange comets.


texts: Why is only a narrow chunk of sky darkened in this amazing Curiosity solar eclipse GIF from Mars?

texts:
I found three NASA JPL images in the sequence PIA23133, PIA23134, and PIA23135. They were taken by the Curiosity Rover on Mars and show solar eclipses by Mars' two moons.

Details of these images are given in this answer to Puzzler: help understanding these amazing Curiosity eclipse GIFs

Question: Why is it that it's mostly a chunk of the right half of the sky that darkens during the eclipse in PIA23135?

Is this kind of effect visible in solar eclipses seen on Earth?

Enquiring minds want to know!

fyi in case you are wondering, per @MarkAdler's ultra-concise answer

  They use neutral-density filters to look at the Sun, which reduce the light by a factor of 100,000. The two Pancam cameras each have one neutral-density filter, with the left one filtering blue and the right one filtering red.

From https://photojournal.jpl.nasa.gov/archive/PIA23133.gif and

From https://photojournal.jpl.nasa.gov/archive/PIA23134.gif

From https://photojournal.jpl.nasa.gov/archive/PIA23135.gif

(a fairly slow) GIF:


texts: Why aren't ground-based observatories using adaptive optics for visible wavelengths?

texts:
Adaptive Optics (AO) techniques allow ground based observatories to dramatically improve resolution by actively compensating for the effects of Astronomical Seeing.

The atmospheric effects are quite variable in both time and location. A parameter called Isoplanatic Angle (IPA) is used to express the angular extent over-which a given wavefront correction optimized for one point (usually a guide star, artificial or natural) will be effective. As an example, Table 9.1 in this Giant Magellan Telescope resource shows values for IPA scaling almost linearly (actually: $\sim\lambda^{6/5}$)  from 176 arcseconds at a wavelength of 20 microns to only 4.2 arcseconds at 0.9 microns.

This suggests an IPA of 2 to 3 arcseconds for visible wavelengths, which taken by itself is not a killer limitation.

However, it seems almost all currently active AO work is done exclusively in various infrared wavelengths, apparently down to 0.9 microns but no further. (AO is also implemented computationally to array data in radioastronomy.)

Is this because the observed wavelength needs to be longer than the guide star monitoring wavelength? Because it is simply much harder and there is always Hubble above the atmosphere for visible work so it's not worth the extra effort, or is there another more fundamental reason?

I'm not looking for speculation or opinion, I'd like a quantitative explanation (if that applies) - hopefully with a link for further reading - thanks!


texts: What are the odds for any two bodies to collide during the milky way-Andromeda collision?

texts: When the two main galaxies of the local group will collide, it is very unlikely for stars to collide with another, but how unlikely is it in the initial collision? And over the whole merging process?


texts: How can the observable universe shrink in a Big Rip?

texts:
As far as I know, the Big Rip occurs when the scale factor reaches infinity in a finite time. This will only happen in a universe dominated by phantom energy (i.e. a universe with an equation of state of $< -1$). However, according to Wikipedia,

  A universe dominated by phantom energy is an accelerating universe, expanding at an ever-increasing rate. However, this implies that the size of the observable universe is continually shrinking; the distance to the edge of the observable universe which is moving away at the speed of light from any point moves ever closer. When the size of the observable universe becomes smaller than any particular structure, no interaction by any of the fundamental forces can occur between the most remote parts of the structure. When these interactions become impossible, the structure is "ripped apart".

On Wikipedia, there is a "[why]" tag, indicating that this wasn't explained very well.

This raises two questions. In a universe dominated by phantom energy, why would the observable universe not continue growing and approaching the cosmic event horizon? After all, its comoving distance is just $\int_0^t \frac{\mathrm{d}t}{a(t)}$, so shouldn't it just monotonically increase?

More confusingly, Wikipedia implied that the Big Rip occurs when the observable universe's radius becomes incredibly small and the fundamental forces can no longer hold any structure together, rather than when the scale factor reaches infinity. Am I right in assuming these are separate events, or do they occur at the same time?


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whether its in movies,television,real-life or some planetarium different galaxies are shown in different color.

For ex:- Milky way is always seen in Blue where as Andromeda gives out a bit reddish color.

Is it because different galaxies contain different kinds of stars?
Does this effect the life the galaxy harbors...

^_^


texts: How can I create a mask for a fits image?

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I need to create a mask of a fits image.
By this I mean that I want to create the image that covers all secondary objects from the original image except the one is in the center.
I have some routines in Python+sextractor, but the code a bit dated, is there are some new tools?


texts: What is the speed of light relative to (in space)?

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From what I understand the universe is expanding faster than the speed of light and accelerating so that it's possible that a given galaxy: $A$ will no longer be visible to another given galaxy: $B$.

So, if a light wave is emitted from galaxy $A$ toward galaxy $B$ after it has passed the point where neither galaxy is visible to each other, how can the speed of the light wave be measured if there is no fixed point in space for it to be measure against (as the light wave will never be visible to either galaxy again)?


texts: As the Moon and the Earth are predicted to get into tidal lock, how slow would the Earth rotate?

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This answer to Will the Earth ever be tidally locked to the Moon? supports the widely held thinking that during the Sun's red giant phase or later the Earth and the Moon should be tidally locked to each other. The Earth is said to have a slower rotation then.
Do we have any idea how long a sidereal day and the mean solar day on Earth would be in several billion years?


texts: Would the heliosphere disappears if our sun becomes a white dwarf?

texts: Just curious I like to know if one fine day our Sun turns into a white dwarf at the later stage of its life, would the magnetic bubble, heliosphere gets thinner and eventually disappear? also at this moment is it changing in size?